The processes for the treatment of hydrocarbons carried out in refineries and/or petrochemistry units include a number of treatments carried out in the presence of hydrogen and which are intended to modify the structure of the hydrocarbon molecules and/or to remove undesirable compounds from the hydrocarbon fractions, such as in particular sulphur-comprising, nitrogen-comprising, aromatic or metal compounds. Mention may be made, as non-limiting examples, of hydrocracking, reforming or hydrogenation processes and “hydrotreating” processes, such as hydrodesulphurization, hydrodenitrogenation, hydrodearomatization or hydrodemetallization.
These processes involve specific catalysts which comprise a porous support based on one or more refractory inorganic oxides on which are deposited one or more catalytically active metals comprising at least one metal from Group VIII of the Periodic Table of the Elements, generally in combination with one or more metals from Group VIB.
During their use, these catalysts gradually become deactivated, in particular due to the deposition of coke at their surface, that is to say of a mixture of heavy hydrocarbons, of carbon residues and of metal impurities.
For the sake of economy and preserving the environment, there is increasingly a search henceforth to reuse these catalysts after their cycle of use.
“Regeneration” processes have thus been developed, which consist in treating the spent catalysts in order to restore their activity to a level sufficient to allow them to be reemployed.
The regeneration of spent catalysts is conventionally carried out by combusting the coke, the catalyst being heated to a high temperature in the presence of a gas comprising oxygen. It can be carried out in situ (that is to say, directly in the unit, after shutting it down) or ex situ (that is to say, after discharging the catalyst from the unit).
However, after their first regeneration, these catalysts exhibit an activity which is sometimes markedly inferior to their initial activity in the fresh state.
For this reason, “rejuvenation” processes have recently been developed in which the regenerated catalysts are impregnated with an organic additive with the aim of bringing their activity back to a level as close as possible to that of a fresh catalyst.
Thus, Patent Application WO 96/41848 describes a process for the activation of a hydrotreating catalyst comprising an oxide of a metal from Group VIII and an oxide of a metal from Group VI which are deposited on a support. According to this process, the catalyst is brought into contact with an additive which is a compound comprising at least 2 hydroxyl groups and from 2 to 10 carbon atoms, or a (poly)ether of such a compound, and then the catalyst is dried under conditions such that at least 50% of the additive remains present on the catalyst.
This process can be applied to a fresh catalyst, the activity of which it is desired to increase, or to a spent catalyst, which has been subjected beforehand to a regeneration step. Advantageous additives include ethylene glycol, diethylene glycol and polyethylene glycols.
Patent Application EP 0 882 503 describes a process for the regeneration of a spent catalyst comprising a support based on gamma-alumina and on amorphous alumina impregnated with one or more catalytically active metals, in which:
(1) the spent catalyst is treated in order to remove carbon-based deposits;
(2) the support thus treated is wetted using a chelating agent in a supporting liquid;
(3) the support, thus wetted, is subjected to an ageing phase;
(4) the support is dried, so as to evaporate the supporting liquid;
(5) the support, thus dried, is calcined.
The chelating agents cited are ethylenediaminetetraacetic acid (EDTA) and its derivatives, such as, for example, N-hydroxy-EDTA and diammonium-EDTA, tri(2-aminoethyl)amine, triethylenetetraamine, diethylenetriaminepentaacetic acid, cyclohexane-diaminetetraacetic acid, ethylene glycol bis(β-aminoethyl ether) N,N′-tetraacetic acid and tetraethylenepentaamine.
Patent Application WO 01/02092 describes a process for regenerating and rejuvenating a spent additivated catalyst comprising a step of regeneration of the catalyst by bringing it into contact with an oxygen-comprising gas at a maximum temperature of 500° C. and then a step of rejuvenation of the catalyst by bringing the latter into contact with an organic additive, optionally followed by drying at a temperature such that at least 50% of the additive remains present on the catalyst.
The regeneration temperature is preferably between 350 and 425° C. The organic additive employed in this process can be any compound comprising at least one carbon atom and one hydrogen atom.
However, the methods described in the prior art exhibit a number of disadvantages. In particular, some of the additives employed in these methods exhibit a relative toxicity which makes them more difficult to employ, especially on an industrial scale. It has also been found that some of these additives, once deposited on the catalyst, subsequently result in release of carbon monoxide (CO), which is a particularly toxic and polluting gas.